Molecular sieves are a commercially important class of crystalline materials. They have distinct crystal structures with ordered pore structures which are demonstrated by distinct X-ray diffraction patterns. The crystal structure defines cavities and pores which are characteristic of the different species. Molecular sieves such as zeolites have been used extensively in catalysis, adsorption, separation, and chromatography.
Molecular sieves are classified by the Structure Commission of the International Zeolite Association according to the rules of the IUPAC Commission on Zeolite Nomenclature. According to this classification, framework type zeolites and other crystalline microporous molecular sieves, for which a structure has been established, are assigned a three-letter code and are described in the “Atlas of Zeolite Framework Types” Sixth Revised Edition, Elsevier (2007).
Molecular sieves having a MTT-type framework code have a one-dimensional 10-ring pore system. MTT-type molecular sieves have very similar, but not identical, X-ray diffraction patterns. SSZ-32 and its small crystal variant, SSZ-32x, are known MTT-type molecular sieves.
SSZ-32 and methods for making it in the presence of N-lower alkyl-N′-isopropylimidazolium cations are disclosed in U.S. Pat. Nos. 5,053,373 and 5,252,527. SSZ-32 is reported to have a SiO2/Al2O3 molar ratio in a range of 20 to less than 40.
SSZ-32x and methods for making it in the presence of N-lower alkyl-N′-isopropylimidazolium cations are disclosed in U.S. Pat. Nos. 7,390,763 and 8,545,805. SSZ-32x is reported to have a SiO2/Al2O3 molar ratio in a range of 20 to less than 40 with crystallites having small broad lathe-like components in a range of 20 to 40 nm.
SSZ-32x, in comparison with standard SSZ-32, has broadened X-ray diffraction peaks that may be a result of its inherent small crystals, altered argon adsorption ratios, increased external surface area and reduced cracking activity over other intermediate pore size molecular sieves used for a variety of catalytic processes.
According to the present disclosure, it has now been found that dipropylamine is effective as a structure directing agent in the synthesis of high-silica forms of SSZ-32x in fluoride media.